1) Field of the Invention
The present invention relates to a lubricant applying unit that applies lubricant to form a thin layer of the lubricant, a process cartridge that performs image formation by electrostatic copying and an image forming apparatus employing the lubricant applying unit, and an image forming method.
2) Description of the Related Art
In an electrophotographic image formation, an image (electrostatic latent image) of electrostatic charge is formed on a photosensitive drum by using a photoconductive developing, and charged fine particles (toner) that are colored are adhered by electrostatic force on the electrostatic latent image to obtain a visualized image. In an electrophotographic image forming apparatus, a technology of applying a material such as a wax, a fluorine-contained resin (such as polytetrafluoroethylene and polyvinylidene fluoride), and a metal salt of high fatty acid (zinc stearate) as a lubricant on main components such as the photosensitive drum and an intermediate transfer belt, is available. This technology is used for eliminating trouble in a cleaning process in the electrophotography or in other words in scraping toner remained on the photosensitive drum or the intermediate transfer belt by a cleaning brush or a cleaning blade.
One of the problems is an increase in a life of the photosensitive drum and the intermediate transfer belt. It has been revealed that the life of the photosensitive drum and the intermediate transfer belt is determined mainly by wearing out due to mechanical rubbing of the cleaning brush and the cleaning blade. Therefore, by applying a lubricant on the photosensitive drum or the intermediate transfer belt, a coefficient of friction of the surface of the photosensitive drum or the intermediate transfer belt is allowed to be lowered, thereby reducing the wearing out.
Another problem relates to the cleaning. By applying the lubricant on the surface it is possible to reduce the coefficient of friction of the surface and to remove easily the deposits adhered to the surface. In other words, it is possible to remove easily the toner remained on the photosensitive drum and the intermediate transfer belt that has not been transferred fully to a final recording medium such as paper. In recent years, a use of a spherical toner that is prepared by a polymerization is started. In such a toner, since a particle-size distribution is uniform and the size of particles is controlled efficiently to a small size, there is an improvement in the image quality, however the cleaning of the toner remained on the photosensitive drum becomes difficult. In view of such a technological background, the improvement in cleaning by using the lubricant is supposed to become even more important as a technology.
A small of amount of the lubricant in the form of fine particles is supplied to the surface of the photosensitive drum and a concrete method of scraping the lubricant in the form of a solid block and applying by using an applying unit such as a brush, has been disclosed in Japanese Patent Application Laid-open Publication No. 2000-162881. A method of supplying the lubricant to the photosensitive drum by adding it externally to the toner has been disclosed in Japanese Patent No. 2859646. However, when the lubricant is added externally to the toner and supplied to the photosensitive drum, an amount of the lubricant supplied depends on an image area that is output and since it is not possible to supply the lubricant to the overall surface of the photosensitive drum, a method of scraping the solid lubricant by a brush and applying on the surface of the photosensitive drum is desirable to supply the lubricant stably to the overall surface of the photosensitive drum with a simple structure of a unit.
However, in a case of applying the lubricant on the photosensitive drum, an amount to be applied and a control of the application is very important for exerting the effects of the prevention of wearing out and the improvement in cleaning. If a lubricating ability is emphasized, a lubricant not less than a certain fixed amount may be supplied continuously. However, it has been revealed that if the lubricant is supplied excessively, the lubricant enters into a developer unit and an amount of charging of the toner cannot be controlled. Moreover, from a trend of reduction in a size of copying machines and printers in recent years, reducing the size of each component has become an important technique and it is not desirable to load in the unit the solid lubricant more than that is required.
Various methods of controlling the application of the lubricant have been proposed so far. According to Japanese Patent Application Laid-open Publication No. H9-62163, a welding pressure of the solid lubricant on the photosensitive drum or a rotational speed of an applicator-brush in contact with the solid lubricant is controlled according to a temperature environment. According to Japanese Patent Application Laid-open Publication No. 2000-75752, an amount of lubricant applied per unit number of revolutions of the photosensitive drum is regulated. According to Japanese Patent Application Laid-open Publication No. 2002-24485, parameters such as the number of revolutions of the applicator brush are controlled in accordance with image forming information.
To solve these problems, the lubricant is applied with object of preventing occurrence of toner filming (fusing), preventing improper cleaning, and improving efficiency of transferring by lowering the coefficient of friction. Inventions disclosed in Japanese Patent No. 2859646, Japanese Patent Application Laid-open Publication Nos. H9-62163, 2000-75752, and 2002-24485 have been known and the problems are solved by applying the lubricant on the photosensitive drum and lowering the coefficient of friction.
To increase the life of a charging member and the photosensitive drum, a technology in which a non-contact charging unit is used, inorganic fine particles are allowed to be dispersed on a photosensitive layer of the photosensitive drum, and a compound such as zinc stearate is applied as a lubricant to improve a wear resistance has been disclosed in Japanese Patent Application Laid-open Publication No. 2002-229227.
In addition to this, according to a technology disclosed in Japanese Patent Application Laid-open Publication No. H10-142897, an example of an image forming apparatus in which the lubricant that is applied on the surface of the photosensitive drum is allowed to be deposited uniformly as a thin layer between the charging member and a developing section and has an auxiliary member in the form of a blade to hold back the lubricant having a large diameter, has been mentioned.
Still another problem is that the lubricant cannot be applied sufficiently and uniformly on the surface of the image carrier and a part of the surface of the image carrier goes on wearing out. To solve this problem, various methods of controlling the application of the lubricant have been proposed so far.
According to Japanese Patent Application Laid-open Publication No. H11-38855, a method of applying the lubricant evenly on the surface of the image carrier, in which, upon completion of the image forming operation, the photosensitive drum is rotated in a reverse direction and a thin film of the lubricant is applied by dragging the lubricant stored on the cleaning blade along the surface of the photosensitive drum, has been disclosed.
According to Japanese Patent Application Laid-open Publication No. 2001-343861, for providing an image forming apparatus that enables to prevent a temporary decrease in an amount of consumption of the lubricant initially when a process cartridge is replaced by a new one and to obtain a good image right from the beginning of the output, the image carrier, a lubricant applying unit, and a toner holding-back member are integrated to form a process cartridge that is detachable from a main body of the apparatus, and a substance supplying unit that supplies to the surface of the image carrier a substance that can exert an adsorptive power on the lubricant is provided. When the process cartridge is replaced by a new one, the lubricant is applied on the surface of the image carrier till the image forming operation is started and the lubricant applying unit and the substance supplying unit are controlled such that the substance is supplied to the surface of the image carrier.
However, according to the above method, the lubricant is attracted to the substance that exerts the adsorptive force and cannot be supplied appropriately to the image carrier.
According to Japanese Patent Application Laid-open Publication No. 2002-244487, in an image forming apparatus in which a small particle size toner or a fine particulate toner by polymerization is used, even if the developing cartridge is replaced, a proper amount of the solid lubricant is applied on the surface of the photosensitive drum to provide a highly reliable image forming apparatus in which improper cleaning and fusion do not occur during a durable life. According to this method, in an image forming apparatus that includes a lubricant-film forming unit that forms a film of the lubricant on the surface of the image carrier by bringing in contact the solid lubricant with the rotating brush and applying the lubricant that is adhered to the brush on the image carrier and a developing unit that develops by toner the electrostatic latent image formed on the image carrier, the developing unit is included in the cartridge that is detachable from the apparatus and the amount of the lubricant to be applied on the surface of the image carrier is controlled based on information of condition in which the cartridge is used.
According to Japanese Patent Application Laid-open Publication No. 2003-36011, a cleaning unit in which the rotational speed of the cleaning brush in contact with the photosensitive drum is adjusted appropriately to prevent the occurrence of improper cleaning has been proposed. According to this method, the cleaning unit in which the toner remained on the photosensitive drum after an image is formed by transferring a toner image formed on a rotatable photosensitive drum by at least two circumferential velocities viz. a first circumferential velocity and a second circumferential velocity that is faster than the first circumferential velocity, is structured such that during the image forming, when the photosensitive drum rotates at a first circumferential velocity VA the cleaning brush is allowed to rotate at a first circumferential velocity VB and when the photosensitive drum rotates at a second circumferential velocity VA′, the cleaning brush is allowed to rotate at a second circumferential velocity VB′ and the relationship of VA, VB, VA′, VB, and VB′ is VA<VA′ and (VB/VA)>(VB′/VA′).
Regarding the amount of the lubricant to be applied on the photosensitive drum, a technology disclosed in Japanese Patent Application Laid-open Publication No. 2000-338733 according to which the amount of the lubricant to be applied and a charging potential are controlled based on a result of detection of density of a pattern image formed on the photosensitive drum and a technology disclosed in Japanese Patent Application Laid-open Publication No. 2003-330320 according to which the amount of the lubricant to be applied is controlled according to conditions such as degree of wearing out of the cleaning blade, number of images formed, a distance covered by the photosensitive drum, and a temperature of the blade, have been proposed.
However, according to Japanese Patent No. 2859646, and Japanese Patent Application Laid-open Publication Nos. 2000-162881, H9-62163, 2000-75752, and 2002-24485, the application of the lubricant has been controlled indirectly and not controlled by observing directly the application on the photosensitive drum or the intermediate transfer belt. According to Japanese Patent Application Laid-open Publication Nos. H9-62163 and 2002-24485, the number of revolutions of the brush and the welding pressure of the solid lubricant are controlled and it is not clear as to how the lubricant is applied practically on the photosensitive drum and the intermediate transfer belt. Moreover, according to Japanese Patent Application Laid-open Publication No. 2000-75752, although the amount of the lubricant to be applied is regulated, this amount of the lubricant, to be more precise, is the amount of the solid lubricant consumed and not the amount of the lubricant on the photosensitive drum. This is because, in the method of applying the lubricant by the brush, the lubricant in the form of a powder is always adhered to the brush and when the lubricant is transferred from the brush to the photosensitive drum, a part of it is shaken off.
So far, powdered particles scattered as the lubricant on the photosensitive drum have been observed but no attention has been paid to observation and periodic evaluation of a lubricant other than that in the powder form applied on the photosensitive drum.
According to the technology disclosed in Japanese Patent Application Laid-open Publication No. H1-38855, the lubricant cannot be applied sufficiently and uniformly, and the object has not been achieved. Moreover, according to the invention disclosed in Japanese Patent Application Laid-open Publication No. 2002-244487, although the amount of the lubricant to be supplied to the image carrier can be increased and decreased, it is difficult to apply the lubricant uniformly on the image carrier.
According to the technology disclosed in Japanese Patent Application Laid-open Publication No. 2003-36011, although the amount of lubricant to be supplied to the image carrier can be increased and decreased, it is difficult to apply the lubricant uniformly on the image carrier.
The photosensitive drum is subjected to various hazards during the process of image formation and so is the lubricant that is applied on the photosensitive drum. A process of charging the surface of the photosensitive drum, which precedes the formation of the electrostatic latent image on the surface of the photosensitive drum, is one of the factors that cause such hazard. Such a hazard depends a lot on a method of charging that is used in the image forming apparatus. For the lubricant to perform fully its function, the amount of the lubricant to be applied is to be determined upon having taken into consideration the method of charging, which is not known to have been done so far.
The methods of charging in the electrophotographic image forming apparatus include methods such as corona charging and contact charging or proximity charging. Normally, in the contact charging and the proximity charging, an amount of a product material discharged is less and the charging can be performed at low electric power as compared to that in the corona charging. However, on the other hand, in the contact charging and the proximity charging, it has been revealed that since the photosensitive drum is in contact with a charging member or a distance between the photosensitive drum and the charging member becomes shorter than that between the photosensitive drum and a charging wire, the hazard caused to the photosensitive drum is more than that in the corona charging. Particularly, when AC voltage is superimposed, a corona discharge is repeated according to a frequency of the AC voltage, thereby letting the hazard to be greater. As a result, the surface of the photosensitive drum is deteriorated chemically and in a due course of time a film on the photosensitive drum is scraped. If the lubricant is applied on the surface of the photosensitive drum, there is a change in a surface energy and a molecular structure of the lubricant, thereby resulting in loss of the lubricant. The lubricant is scraped gradually and finally vanishes.
Therefore, initially, even if an appropriate amount of the lubricant is applied on the photosensitive drum, for the lubricant to be stabilized upon elapsing of time and to perform its function, the lubricant is to be continued to be applied considering the chemical deterioration of the lubricant during charging.